Diffusion bonding is an extremely valuable technique in joining components, particularly in the aircraft industry and involves the pressing together of heated components so that the atoms in the components interdiffuse to form a metal-to-metal bond. Diffusion bonding can in the case of certain metals having a specific uniform grain structure (e.g. titanium) be combined with superplastic forming, which is a technique in which a metal article, usually in sheet form, is heated and subjected to slow deformation during which the metal stretches and is thinned out in the deformed areas but does not neck or fracture and in this way the metal article can be formed into a desired shape.
Aluminum and many of its alloys can be formed superplastically but they have an extremely tenacious surface oxide that prevents diffusion bonding; because of the physical properties of aluminum (low density and high strength), it is ideal for use in the aircraft industry but its inability to be formed into a composite structure by diffusion bonding has caused design limitations. The term "aluminum" will be used in this Specification to include both aluminum and aluminum alloys.
In our earlier British Patent Application No. 8815663.3 (corresponding to EP-A-0 350 220, U.S. Ser. No. 373,492 now U.S. Pat. No. 4,948,457), we described a method of diffusion bonding a component made of aluminum to a further component (which may also be made of aluminum) by subjecting the aluminum component(s) to grit blasting and to a chemical treatment to remove the oxide layer followed by assembling the components into a stack (or pack) in which the components are positioned in the configuration desired for the final diffusion-bonded article, and finally diffusion bonding the assembled stack at a temperature preferably of 540.degree. to 580.degree. C. for two to five hours. The diffusion bonding step is performed by placing the components to be joined (which are usually sheets) in a cavity, placing a superplastic membrane over the cavity, compressing the edges of the membrane against the cavity wall so as to seal the cavity, applying a gas pressure on the side of the membrane facing away from the cavity and simultaneously evacuating the cavity, whereby the differential pressure across the membrane compresses the components in the cavity. The components are heated to 540.degree. to 580.degree. C. which together with the pressure exerted on the components as a result of the pressure differential across the membrane causes the desired diffusion bond to be formed between the components. It is necessary to evacuate the cavity during diffusion bonding, e.g. to a pressure of 10.sup.-6 mbar, in order to prevent the oxide layer on the aluminum component from reforming to such an extent that it blocks diffusion of the metal atoms and hence prevents diffusion bonding; the formation of some oxide is inevitable but if it is limited then the resulting diffusion bonding can have adequate strength; we have found that it is necessary, if the bond is to have an acceptable strength, for the diffusion bonding to be performed within about 20 minutes of the grit blasting/chemical treatment step.
The technique described above has a number of drawbacks:
(1) High vacuum levels are required for successful diffusion bonding and this means that the membrane must provide an excellent seal and also the vacuum pumping equipment must be of a very high quality in order to ensure that an adequate vacuum is maintained within the cavity. Whereas this can be adequately achieved on a laboratory-scale, it is extremely difficult and expensive to achieve on an industrial-scale,
(2) Since the cleaned surfaces of the aluminum component can only be exposed to air for a maximum of 20 minutes, the assembly of the stack and the evacuation of the cavity must be carried out within this period; as will be appreciated, this is a difficult task to perform, particularly on an industrial scale, and
(3) The monitoring of the pressure within the cavity is extremely difficult and it is possible to trap pockets of air between sheets of material to be bonded together and such air pockets contaminate the surfaces being bonded.
We have now discovered a technique whereby the above disadvantages can be overcome or at least reduced.
EP-A-0 022 605 and EP-A-0 058 569 both describe methods of bonding dissimilar metals to each other by subjecting them to extensive working to form a composite article with an exterior layer of a corrosion-resistant material on top of a base material providing strength to the article and/or different corrosion resistance. Examples given in EP-A-0 022 605 are of making tube or sheet having a base of ordinary steel and an exterior covering layer of stainless steel. Prior to the working step the layers of dissimilar metals are welded by electron beam welds primarily to prevent the layers from exfoliating during the working step but also to prevent oxygen from contacting the interface between the layers during hot working which would if present inhibit the formation of the bond between the layers.